Thermoplastic copolymer of tetrafluoroethylene and perfluoromethyl vinyl ether and medical devices employing the copolymer

ABSTRACT

An improved elastomeric material is described comprising an essentially noncross-linkable amorphous copolymer of tetrafluoroethylene (TFE) and perfluoromethyl vinyl ether (PMVE) which is both a thermoplastic and exhibits exceptional mechanical properties. This material is particularly suitable for use in ultra-clean environments, and particularly for use in an implantable device, since it does not contain contaminants that previous thermoset TFE/PMVE copolymers have required. Among the improved properties of the present invention are excellent biocompatibility, high matrix tensile strength, high clarity, high abrasion resistance, high purity, adequate elasticity, and ease of processing due to the thermoplastic, and noncross-linkable structure of the copolymer. The material of the present invention is also a high strength bonding agent particularly suited for bonding porous PTFE to itself or to other porous substances at room or elevated temperatures.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of application Ser. No. 10/382,157filed Mar. 4, 2003, which is a continuation of application Ser. No.09/653,210 filed Aug. 31, 2000 (abandoned) which is a continuation ofapplication Ser. No. 09/233,368 filed Jan. 19, 1999 (abandoned).

FIELD OF THE INVENTION

The invention relates to a thermoplastic noncross-linkable copolymer oftetrafluoroethylene and perfluoromethyl vinyl ether and medicalapplications of the copolymer.

BACKGROUND OF THE INVENTION

Fluorinated polymers are well known for their physical properties suchas corrosion resistance, low coefficients of friction, chemicalresistance, and thermal stability. An example of such a fluoropolymer ispolytetrafluoroethylene (PTFE), which is widely used in medical andindustrial applications. PTFE has an extremely high melt viscosity,which makes processing by melt fabrication or injection molding verydifficult. Full density PTFE is also generally non-elastic orsemi-rigid, making it unsuitable for applications requiring some degreeof elasticity.

Elastomeric fluoropolymers, such as KALREZ® elastomer available from E.I. duPont de Nemours & Co., combine the advantages of a fluoropolymerwith the mechanical aspects of an elastomer. The mechanical andelastomeric properties of such materials are, however, a result of thecross-linking or curing, which improves strength and recovery, andrenders the polymer a thermoset. The incorporation of a cross-linkingsystem, which improves elastomeric properties of the thermoset, requiresadditional monomers to be added during polymer synthesis, andcross-linking agents to be added during milling and compounding steps.The compound is subsequently molded and heat treated to form the finalarticle. This heat treatment is also known as curing or vulcanization.Biocompatibility of the final product is compromised by toxic agents andadditives necessary for cross-linking. heat treatment is also known ascuring or vulcanization. Biocompatibility of the final product iscompromised by toxic agents and additives necessary for cross-linking.

In U.S. Pat. No. 3,132,123 to Harris et al., assigned to E. I. Du Pontde Nemours & Co. Inc., a semi-crystalline copolymer oftetrafluoroethylene (TFE) and perfluoromethyl vinyl ether (PMVE) isdisclosed. This copolymer of TFE and PMVE was prepared in an attempt todevelop a plastic material having the desirable physical properties ofPTFE without the undesirable high melt viscosity, which makes meltprocessing of the polymer very difficult. Thus, these efforts focused onthe development of a melt-processable, injection moldable PTFEsubstitute. The copolymer disclosed had low levels of PMVE(approximately 11% by weight), which is sufficient to yield a meltviscosity lower than PTFE but not sufficient to make the polymeramorphous. This, in turn, would enhance the ease of fabrication byinjection molding. The biocompatibility of this copolymer was notaddressed nor suggested.

Subsequent development efforts relating to the copolymer of TFE and PMVEfocused on cross-linking systems, for the purpose of enhancing themechanical properties and imparting elastomeric properties to thepolymer. In publications by Du Pont, Kalb et al., “A New EngineeringMaterial for Advanced Design Concepts,” Applied Polymer Symposium No.22, 127-142 (1973), and Barney, et al., “A High-Performance FluorocarbonElastomer,” Journal of Polymer Science, Part A-1, Vol. 8, 1091-1098(1970), the need as well as the difficulty of cross-linking the TFE andPMVE polymer is disclosed. These publications focus on the search for athird monomer, to be copolymerized with the TFE and PMVE, that wouldprovide a site for vulcanization or chemical cross-linking. Barney, etal., discloses a tensile strength of 675 psi for the uncured,noncross-linked terpolymers in the form of a gum. These and similardevelopment efforts led to the commercialization of KALREZ® elastomer,which is a thermoset, cross-linked terpolymer, containing monomers ofTFE, PMVE and additional other monomers. Similar thermoset,cross-linked, terpolymers have been developed and are commerciallyavailable under the trade names CHEMRAZ® and DAI-EL PERFLUOR®. Althoughpossessing good mechanical properties, these cross-linked systems arenot suitable for medical applications, particularly implantable medicaldevices, due to the known toxicity of the additives.

All known copolymers containing TFE and PMVE comonomers commerciallyavailable in final form (e.g., O-rings, sealants and gaskets) arecross-linked materials containing additives and fillers. Noncross-linkedresins or articles of these copolymers (generally terpolymers) aretypically in the form of gums. These gums generally have low molecularweight and poor mechanical properties. They are not useful untilcross-linked. The lack of commercially available resins of amorphousthermoplastic copolymers of TFE and PMVE has hindered thoroughbiocompatibility testing of this copolymer. To the knowledge of thepresent inventors, there have been no publications citing efforts totest or determine the biocompatibility of a noncross-linkable,thermoplastic copolymer of TFE and PMVE. There have only beengeneralized references relating to the biocompatibility of plastics madefrom copolymers of TFE and a perfluoroalkyl-vinyl-ether, or aperfluoroalkoxy-vinyl-ether (see, e.g., Homsy, “Biocompatibility ofPerfluorinated Polymers and Composites of these Polymers,”Biocompatibility of Clinical Implant Materials, Volume II, Chapter 3,pp. 59-77 (1981)).

Presently there is dearth of suitable implantable elastomeric materials.Currently available elastomers in this field are predominantly siliconesand polyurethanes, which have well documented deficiencies related tolong term stability in-vivo and mechanical weakness. These deficienciesmay include adverse foreign body reactions, biological response toleachable species, particulation concerns, long-term embrittlement andstress cracking. In addition, these polymers are thermosettingelastomers, which have known limitations in processability. Disclosed inU.S. Pat. No. 4,816,339 to Tu et al. are articles of expanded PTFE(ePTFE) combined with thermoset elastomers. Various thermoset elastomersare disclosed including copolymers of TFE and PMVE in cross-linked orcured forms. These elastomers are believed to suffer from similardeficiencies as those outlined above.

Thus it would be desirable to provide a perfluoropolymer in a pure,noncross-linkable, thermoplastic, amorphous form. Such aperfluoropolymer could ideally have elastic properties, high tensilestrength, high purity, excellent clarity and abrasion resistance alongwith ease of processing.

SUMMARY OF THE INVENTION

The copolymer of the present invention is a copolymer oftetrafluoroethylene (TFE) and perfluoromethyl vinyl ether (PMVE) that isuniquely formed to have excellent mechanical properties while beingsubstantially noncross-linkable, i.e., free of cross-linking monomersand curing agents. “Noncross-linkable” as used herein means that thecopolymer is free of cross-linking monomers and curing agents. Such anamorphous and noncross-linkable material is believed to be particularlyuseful as a biomaterial. The copolymer contains between 40 and 80 weightpercent PMVE units and complementally between 60 and 20 weight percentTFE units. The lack of cross-linking systems ensures that the materialis highly pure and, unlike previous thermoset TFE/PMVE elastomers, isideally suited as an implantable biomaterial. Advantages of the presentinvention include excellent biocompatibility, high tensile strength,high clarity, high abrasion resistance, high purity, adequateelasticity, and ease of processing due to the thermoplastic andnoncross-linkable structure of the copolymer. The copolymer of thepresent invention is thermoplastic and amorphous. It also is of highstrength and can be used as a bonding agent particularly suited forbonding porous PTFE to itself or to other porous substances at room orelevated temperatures. It may also be used to bond nonporous materialsincluding polymers such as nonporous PTFE.

The thermoplastic copolymer of the present invention is particularlyuseful as an advanced material for medical applications. Thenoncross-linkable form of the thermoplastic, poly(TFE-co-PMVE), displayssurprisingly high tensile strengths. Tests demonstrate tensile strengthsof over 90 MPa, compared to the typical published strengths of about 10MPa or less for prior noncross-linked TFE/PMVE copolymers. Similarly,the biocompatibility data derived by chronic in vivo testing displaysexcellent results, as well as unexpectedly positive results regardingoptical clarity and abrasion resistance. The lack of cross-linkingagents in the copolymer of the invention allows the material to bereadily purified to extremely low contaminant levels. Samples of thematerial of the present invention have exhibited contamination levels ofless than 30 parts per billion.

The copolymer of the present invention can be used in a wide variety ofmedical and commercial applications. Medical applications include theincorporation of the inventive copolymer into long and short termimplantable devices, as well as in disposable, or single use, suppliesand devices. These applications include, but are not limited to,vascular grafts, surgical sheets, catheters, space filling oraugmentation devices, joint spacers, surface coatings for devices,lenses, work surface or clean room surface coatings, seals, gaskets,blood contact surfaces, bags, containers, fabric liners, andendoprostheses. In addition, the copolymer of the present invention canbe used as a biocompatible bonding agent for fluoropolymers. Commercialapplications include, but are not limited to, corrosion resistant linersfor vessels or ducts, sealing members, inflatable devices, filters,membranes, fabric liners and as a fluoropolymer bonding agent. Inaddition the thermoplastic characteristics of the present inventionallow the re-melting, reprocessing and recycling of various articles.The material can also be used with various fillers to particularlyinclude therapeutic agents or drug delivery.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 describes a process flow depicting a preferred process ofmanufacture for the copolymer of the present invention.

FIG. 2 describes an alternate process of manufacture for the copolymerof the present invention.

FIG. 3 describes a conventional process for the manufacture of typicalthermoset articles.

FIGS. 4A through 4F show typical NMR profiles or signatures for thecopolymer of the present invention. Shown are various profiles forrandom samples of the copolymer of the present invention in normal andhigh resolutions.

FIG. 5 shows a partial cross section of the mold used for thepreparation of FTIR samples.

FIG. 6 shows a typical FTIR profile or signature of the copolymer of thepresent invention.

FIG. 7 shows a typical visible light transmission profile for materialsof the present invention.

FIG. 8A shows a partial cross section of the sample fixture used in theabrasion wear test.

FIG. 8B shows an isometric view of the abrasion wear test equipment.

FIG. 9A shows a partial cross section of the water leak rate testequipment.

FIG. 9B shows a top view of a water leak rate test sample.

FIG. 9C shows the suturing details of the water leak rate test sample.

FIGS. 10A through 10E show cross sectional views of various laminatestructures.

FIG. 10F shows a partial cross section of a fluid container.

FIG. 11A shows a cross sectional view of a lens.

FIG. 11B shows a partial cross sectional view of a lens.

FIGS. 11C and 11D show two different cross sectional views, depictingthe use of the copolymer of the present invention as a bonding agent.

FIGS. 12A through 12F show isometric views of tubular devices made from,or incorporating, the copolymer of the present invention. Shown aresingle and multi-lumen devices, multilayered devices and solid tubulardevices.

FIGS. 13A and 13B show partial cross sectional views of a mesh deviceand a mesh device incorporated onto a substrate.

FIG. 13C shows a partial cross section of a substrate having a surfacecoating of the copolymer of the present invention.

FIGS. 13D and 13E show partial cross sections of porous embodiments ofthe copolymer of the present invention.

Shown in FIGS. 14A and 14B are porous, open and closed cell embodimentsof the copolymer of the present invention.

FIG. 14C shows a partial cross section of the copolymer of the presentinvention, having a material impregnated within.

FIG. 15 shows a partial cross section of a material, having a node andfibril structure, imbibed with the copolymer of the present invention.

FIG. 16 shows a side view of a bone joint spacer having stemsincorporating the copolymer of the present invention.

FIG. 17A shows a end view cross section of a support structure coatedwith the copolymer of the present invention.

FIGS. 17B and 17C show partial cross sectional views of supportstructures externally or internally coated with the copolymer of thepresent invention.

FIG. 18 shows a graph relating the stress and strain to break relationof the copolymer of the present invention.

FIG. 19 shows a graph relating the stress and strain to 100% relation ofthe copolymer of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The method provided herein results in making an implantable,thermoplastic, non-curable form of poly(TFE-co-PMVE). The methodprovides an ultrapure, elastomeric thermoplastic, particularly suitedfor use in the medical applications and devices of the invention.Currently available elastomers in this field are predominantly siliconesand polyurethanes, which have well documented deficiencies related tolong term stability in vivo and mechanical weakness. These deficienciesinclude adverse foreign body reactions, biological response to leachablespecies, particulation concerns, long term embrittlement and stresscracking. In the present invention, a copolymer material is providedwhich is readily and economically processed to ultrapure levels, ishighly biocompatible, displays elastic properties, has good tensileproperties, and is a thermoplastic. This copolymer material is,therefore, ideally suited for medical grade applications including longterm implants.

The copolymer of the present invention comprises a substantiallynoncross-linkable, amorphous copolymer of tetrafluoroethylene (TFE) andperfluoromethyl vinyl ether (PMVE). This copolymer is formed withoutcross-linking monomers or curing agents that are normally used to formTFE/PMVE copolymers as thermoset elastomers. As a result, the amorphousTFE/PMVE copolymer of the present invention comprises a uniquethermoplastic elastomer, with all the benefits that a thermoplasticmaterial can provide. Surprisingly, however, the material of the presentinvention has remarkably good mechanical properties, including tensilestrength and hardness properties exceeding any previously reportednoncross-linked TFE/PMVE material. Moreover, since cross-linking is notrequired, the copolymer of the present invention can be formed withoutthe addition of cross-linking agents that can lead to device toxicity.The result is a new elastomeric copolymer that is particularly suitablefor use as a biomaterial.

Due to the thermoplastic nature of this copolymer, it is possible tofurther process the polymer powder by a multitude of conventionalmethods. These processing methods include, for example, hot melt screwextrusion, heat compression, vacuum forming, calendering, variouslaminating processes, film or filament winding, dispersion spraying ordipping and injection molding. The polymer may also be rendered porousby methods such as the inclusion of foaming agents, dissolvingimpregnated particles or by forcing hot gasses through thethermoplastic. Articles further processed by these methods include twomajor groups; 1) articles consisting only of the polymer such as tubingor shapes, and 2) articles comprised of the polymer combined with othermaterials, such as laminated articles and filled articles.

The first group of articles, consisting only of the copolymer, includesbut is not limited to such items as medical grade tubing, high puritysheets, trays, flexible containers, forms such as rods, filters orgaskets, gloves, protective garments, implantable lenses and otherimplantable prosthetic devices such as finger joint replacements ortissue augmentation devices. The use of such devices is not limited tomedical applications. For example these devices may have a variety ofapplications in semiconductor manufacturing, chemical handling,electronic assembly or in any clean room environment. Another propertyof this polymer is the ability to form high strength bonds betweenporous polymers, particularly porous polytetrafluoroethylene. In thistype of bonding application, a polymer solution is simply wetted ontothe surfaces to be bonded, the surfaces are then brought into contact.After the solvent is evaporated off, a high strength bond is formedbetween the surfaces even if the bond is performed at room temperature.The polymer of the invention can be used to impregnate partially orfully into porous polytetrafluoroethylene, especially microporousexpanded polytetrafluoroethylene, to form useful composite materials.

The copolymer of this invention can also be blended with fillers such assilica, titanium dioxide, carbon black, therapeutic agents and the likeor other types of fluorinated polymer resin powders.

The second group of articles, comprising the polymer combined with othermaterials, includes but is not limited to such items as chemicalresistant surface coatings or linings, surface coatings which enhancethe device biocompatibility, sheathing for implantable electrical cablessuch as pacemaker/defibrillator leads and electrical insulationconformable coatings or cable coatings. Other applications include,without limitation, laminated devices such as elastomeric layeredvascular grafts, surgical sheets and dura mater substitutes. Theelastomeric properties of this polymer, when combined with animplantable graft or sheet, can result in reduced suture hole bleedingor leaking and shorter time to hemostasis. The elastomeric propertiescan also be used to impart compliance and resilience into these devices.Again, the use of such devices is not limited to medical applications.For example these devices have a multitude of applications insemiconductor manufacturing, chemical handling, electronic assembly orin any clean room environment.

The term “amorphous” as used herein is defined (from the Polymer ScienceDictionary, Second Edition, Mark Alger (Chapman & Hall)) as a polymer inwhich the molecular chains exist in the random coil conformation; sincethere is no regularity of structure, there is no crystallinity.

Crystallinity can be detected by thermal/calorimetric techniques whichmeasure the latent heat of the melting/freezing transitions. Aconvenient method is by Differential Scanning Calorimetry (DSC).

As the term “thermoplastic” is used herein it defines a polymer thatsoftens when exposed to heat and returns to its original condition whencooled to room temperature. Such a polymer can be made to soften, flowor take on new shapes, without significant degradation or alteration ofthe polymer's original condition, by the application of heat or heat andpressure. In contrast to a thermoplastic polymer, a “thermoset” polymeris hereby defined as a polymer that solidifies or “sets” irreversiblywhen cured. A determination of whether a polymer is a “thermoplastic”polymer within the meaning of the present invention can be made byslowly elevating the temperature of a stressed specimen and watching fordeformation. If the polymer can be made to soften, flow, or take on anew shape, without significant degradation or alteration of thepolymer's original chemical condition, then the polymer is considered tobe a thermoplastic. If only small amounts of material are available itmay be necessary to use a hot stage microscope for this determination.

As the term “elastomer” is used herein it defines a polymer that has theability to be stretched to at least twice its original length and toretract rapidly to approximately its original length when released. Theterm “elastomeric” is intended to describe a condition whereby a polymerdisplays stretch and recovery properties similar to an elastomer,although not necessarily to the same degree of stretch and/or recovery.

The copolymer of the present invention is also highly biocompatible. Abiocompatible material is hereby defined as a material being suited forand meeting the purpose and requirements of a medical device, used foreither long or short term implants or for non-implantable applications.Long term implants are defined as items implanted for more than 30 days.

The thermoplastic copolymer of tetrafluoroethylene and perfluoromethylvinyl ether of the present invention, can by incorporated into, orformed into, a wide variety of devices, particularly into medicaldevices. A medical device is hereby defined as an instrument, apparatus,implement, machine, contrivance, implant, or other similar or relatedarticle, including any component, part, or accessory, which is used inthe cure, mitigation, treatment, or prevention of disease, in man orother animals, or intended to affect the structure or any function ofthe body of man or other animals.

The material of the present invention can be formed or incorporated intolaminates. A laminate is defined as a multilayered device wherein atleast two material layers are at least partially affixed together. Thematerial layers can include the same, similar or different materials.

Microemulsion polymerization enables preparation of stable,monodispersed colloidal dispersions containing particles that aresmaller than particles produced with classical emulsion polymerizationprocesses.

In the preferred formulation of the present invention, an aqueous seededmicroemulsion polymerization procedure is used to manufacture thematerial of the present invention in which colloidal copolymer particlesare produced from tetrafluoroethylene and PMVE monomers.

The aqueous seeded microemulsion polymerization procedure involves thefollowing steps:

-   -   (1) a microemulsion of polymerizable unsaturated liquid PMVE        monomer in water is formed as the seed;    -   (2) tetrafluoroethylene (TFE) gaseous monomers are introduced to        the microemulsion system from gas phase;    -   (3) the seeded monomeric microemulsion is free radical        polymerized by charging free radical initiator to start the        polymerization reaction; and    -   The tetrafluoroethylene and PMVE monomers participate in the        polymerization and produce small particles of polymers.

The PMVE polymerizable liquid monomer used in step (1) forms anoil-in-water microemulsion at the polymerization temperature, which canbe between 0 and 150° C., preferably 40 and 100° C. The microemulsion ofthe polymerizable liquid monomer has an average particle size in therange of 1 to 100 nanometer (0.001 to 0.1 micrometer), preferably 1 to80 nanometers (0.001 to 0.08 micrometer), and most preferably 1-50nanometers (0.001-0.50 micrometers).

In step (3), when the TFE gaseous monomer and PMVE are polymerized, thefinal particles generally contain random copolymers.

The amounts of ingredients employed generally are 0.1-40 weight percent,preferably 0.1-20, of the monomers; 0.1-40 weight percent, preferably0.1-25, of the surfactant; with the remainder water.

To initiate polymerization of the seeded microemulsions described above,the temperature of the monomeric microemulsion is adjusted to between 0and 150° C., preferably 40 to 100° C. Initiators can be water soluble oroil soluble free radical initiators, such as persulfates, azoinitiators, peroxides, or photo initiators which can generate freeradicals by ultraviolet or gamma ray activation. Amount of initiatorspresent can range from 0.01 to 20 percent by weight based on the liquidmonomer content. Co-solvents such as an alcohol, amines or otheramphiphilic molecules, or salt can be employed if desired to facilitateformation of the microemulsion. Introduction of initiators cause thepolymerization of monomers to begin.

Sufficient mixing between liquid and gaseous phases should occur toencourage mass transfer. The polymerization temperature can range from 0to 150° C. and preferably 40 to 100° C. The polymerization is carriedout in a pressure vessel and polymerization pressures can range from 200to 200,000 KPa and preferably 200 to 20,000 KPa. The polymerizationproceeds for 1-500 minutes or until at least 50% of the liquid monomersare converted to polymer. The amount of gaseous monomer used can bemeasured by measuring the pressure in the reactor.

The resulting polymer particle latex has an average particle size ofbetween 1 to 100 nanometers (0.001-0.10 micrometers), preferably 1 to 80nanometers (0.001-0.08 micrometers), most preferably 1 to 50 nanometers(0.001-0.05 micrometers), and a polymer average molecular weight of over10,000, preferably over 50,000. The small particle size provides apolymer system with a number of advantages over systems containinglarger particles. The system is a colloidal dispersion and is usuallyclear rather than turbid. The small particle size aids in producingcoatings of uniform thickness and maintains good gas permeability ofporous substrates. The fluorinated monomer units in the polymer chainaids in increasing the thermal stability, hydrophobicity andoleophobicity of the substrates to which the polymer is applied.

In greater detail, the process steps include vacuuming and purging apressure reaction vessel with TFE gas to remove oxygen; followed by:

-   -   a) combining in the pressure reaction vessel a liquid PMVE with        a surfactant in water, at a temperature and ratio of monomer to        surfactant sufficient to spontaneously form a microemulsion        (usually indicated by the aqueous mixture becoming transparent        or translucent),    -   b) charging gaseous TFE monomer to the reactor, and    -   c) initiating reaction of monomers by addition of a free radical        initiator to the reactor.

The seeded monomeric microemulsions are prepared by mixing water, liquidPMVE monomer, surfactants, and optionally co-solvents or inorganicsalts. In order for the microemulsion to form with monomer in it, themonomer must be in liquid form. When the ingredients and amounts areselected for certain operating temperatures, the microemulsion formsspontaneously. The surfactant can be any organic surfactant, butpreferable is a fluorinated anionic surfactant (e.g., a salt offluorinated carboxylic acids, fluorinated sulfuric acids, fluorinatedsulfates, fluorinated ammonium salts, fluorinated non-ionic surfactants,and the like). The ratio of surfactants to other monomers present willusually be 0.5 to 6 or more (by weight).

The TFE monomer and PMVE monomer are used in amounts in thepolymerization such that the polymer produced contains 20 to 60% byweight of tetrafluoroethylene and complementally 80 to 40% by weight ofperfluoromethyl vinyl ether, determined by NMR or IR (infrared).Preferably these weight percents will be 40 to 70% PMVE and 60 to 30%TFE. At these weight percent ranges the copolymer will be amorphous.

Further, to enhance the purity of the copolymer as shown by the flowchart of FIG. 1, this colloidal dispersion is then formed into acoagulum and extracted. The formation of the coagulum involves thefollowing steps:

1) Weighing and filtering the colloidal dispersion or microemulsion toremove gross particulates.

2) Pouring the dispersion into a mixing vessel.

3) Mechanically agitating the dispersion while optionally adding HNO₃,or an electrolyte such as ammonium carbonate, until the mixture formslumps of coagulum material. Alternatively, freezing methods can also beused to form the coagulum material.

4) Pouring out the effluent, adding deionized water and lightlydispersing the coagulated lumps by further agitation.

5) Filtering the coagulation while adding deionized water and stirringthe coagulum, by use of a filtration funnel, to obtain coagulum.

6) Draining the remaining deionized water so the coagulum can be dried.

The formed coagulum is then extracted by the following steps:

1) Transferring the coagulum into a thimble.

2) Filling an extraction column flask with methanol (MeOH) or optionallyother types of alcohols such as ethanol or isopropyl alcohol or othertypes of organic solvents such as ketones or ethers, adding boilingstones and connecting the flask to a soxhlet extraction apparatus.

3) Placing the thimble and coagulum into the soxhlet apparatus andfitting a condenser onto the extractor.

4) Heating the flask filled with MeOH at a temperature from 20 to 100°C.

5) Letting the extraction run for approximately between 12 and 24 hours.

6) Draining the residual MeOH and refilling the flask with clean MeOH.

7) Letting the extraction run for approximately between 24 and 48 hours.

8) Heating the extraction content in a vacuum oven at 5 to 80° C.,preferably at 60° C., 1 to 70 cm Hg, preferably at 45 cm Hg forapproximately 0.5 to 24 hours, preferably 12 hours.

Alternatively, the extraction process can run for 48 hrs withoutreplacement of MeOH after 24 hrs.

A preferred method of processing the noncross-linkable thermoplasticcopolymer material of the present invention is depicted by the flowchartin FIG. 1. The polymerization of the two monomers, tetrafluoroethylene(TFE) and perfluoromethyl vinyl ether (PMVE) is accomplished by themicroemulsion process as previously described. The resultingmicroemulsion has the two monomers TFE and PMVE, referred to genericallyas “A & B” in FIG. 1. After the coagulation process, a coagulum isformed containing the copolymer of monomers “A & B.” Next the coagulumis purified by an extraction process, resulting in a crumb containingcopolymer of monomers “A & B.” Finally, by the application of heat andpressure the crumb is formed into a thermoplastic article, containingonly copolymer of “A & B,” rendering the polymer noncross-linkable.

FIG. 2 presents an alternate process flow similar to the one shown inFIG. 1, with the exception of the initial polymerization step. In thisalternate process flow, the material of the present invention isprocessed starting with a non-microemulsion or standard, conventionalemulsion. However, it is preferred that the microemulsion polymerizationprocess disclosed above is used.

FIG. 3 is a flow chart detailing prior art process steps conventionallyfollowed. Since all known commercial polymers of TFE and PMVE arethermoset or cross-linked systems, a third monomer, generically referredto as “C” is polymerized along with the monomers “A & B.” This thirdmonomer “C” provides a subsequent site for cross-linking. Aftercoagulation and drying, the resultant crumbs are milled or compounded,during which time a fourth, or cross-linking, agent “D” is added. Thesubsequent application of heat and pressure completes the cross-linkingor curing, resulting in a thermoset article having the four components“A & B & C & D.”

The processes described in FIGS. 1 and 2 have distinct advantages whencompared to the conventional process described in FIG. 3. By onlypolymerizing and coagulating “A & B,” as shown in FIG. 1 or 2, theresultant coagulum is easily and readily purified, for example by asimple extraction process. Conventional methods, as shown in FIG. 3,have a third monomer “C,” which is incorporated during polymerization.The conventional process shown in FIG. 3 also has a milling/compoundingstep, necessary for the addition of the cross-linking agent “D.” Thiscompounding process is designed to shear the polymer, which breaks thepolymer chain into shorter segments and uniformly distributes additivesthroughout the bulk of the polymer. Through compounding, thecross-linking agent “D” is placed in close proximity to thecross-linking sites, found in “C.” This repeated shearing of the polymerby milling is known to reduce the tensile strength of the finalmaterial.

The mechanical and physical properties of the copolymers of the presentinvention have been investigated over the composition range ofapproximately 40 to 70 weight percent PMVE. Within that range, thefollowing relationships have been observed.

1. The tensile strength of compression molded sheets tends to decreasewith increasing % PMVE. The exact relationship is difficult to determineat this time because strength is also highly dependent on molecularweight, and all of the methods currently available for estimatingmolecular weight (intrinsic viscosity and melt zero shear viscosity) arealso dependent on composition.

2. The secant modulus at 100% elongation for the copolymer has beenshown to be a strong function of composition, with values of Es rangingfrom approximately 9 MPa for 44 wt % PMVE units in the polymer down toapproximately 1.5 MPa for 70 wt % PMVE units in the polymer. Secantmodulus does not appear to be strongly affected by molecular weight formaterials tested to date.

3. The ability of the copolymer to recover after being held in astrained state is also a strong function of composition. Values oftensile set after 10 minutes of being held at 100% elongation rangedfrom <1% for high 70% PMVE units in the polymer to approximately 100%for 53 wt % PMVE polymer. Tensile set is also a function of molecularweight, with increasing molecular weight giving lower values of tensileset, but the effect is small in comparison to changes in composition.

4. The density of the copolymer is insensitive to composition over therange of 40 to 70 wt % PMVE. Copolymers of the present invention in anonporous form have typical bulk densities of approximately 2.13±0.02g/cc.

5. The refractive index of the copolymer is also insensitive to changesin composition.

6. The hardness (that is, durometer measurement) of the copolymer issensitive to changes in composition.

With respect to the hardness, generally hardness of a non-foamedembodiment will be greater than about 30 Shore A; still lower values mayalso be possible. Depending upon composition, hardness measurements ofnon-foamed embodiments may comprise greater than about: 35 Shore, 40Shore, 45 Shore, 50 Shore, 55 Shore, 60 Shore, 65 Shore, 70 Shore, 75Shore, 80 Shore, 85 Shore, and 90 Shore. Specific forms of the presentinvention have been produced with a hardness measurement of about 85Shore±5 Shore.

Test Procedures

A number of tests are used to define various properties of the materialof the present invention. These tests are described below. Note thatwhile GC, NMR and FTIR are described below for consideration of, forexample, purity and polymer identification, additionally, ESCA (ElectronSpectroscopy for Chemical Analysis), XPS (X-Ray PhotoelectronSpectroscopy) and/or SIMS (Secondary Ion Mass Spectroscopy) can also beused as well as GC, NMR and FTIR (or any other method conventionallyused for polymer analysis) to identify the presence or lack thereof ofcross-linking monomers and/or curing agents.

Particle Size Determination

A COULTER N4MD particle size analyzer is used. The mean diameter ismeasured using a light scattering method with helium laser at scatteringangles of 90 degrees. Each aqueous dispersion sample is diluted about10,000 times with deionized water before measurement.

Durometer

The hardness of samples can be determined through durometermeasurements. Samples are measured at room temperature (about 23° C.) bythe method of ASTM D2240-91 using a Shore Durometer Type 0 with a Shoremodel CV-71200 Conveloader (Shore Instrument Co., Freeport, N.Y.). Thedurometer uses a hemispherical indenter of 1.2 mm radius. Samples testedby this method should be at least 6 mm thick. Two or more samples may bestacked if necessary to achieve the minimum 6 mm thickness. Fivedurometer readings should be taken at five different points on eachsample; these five readings are then averaged with the resulting meanvalue taken as the representative hardness value of the sample.Thickness measurements are the average of three or more measurementswith a set of measuring calipers.

Matrix Tensile Strength

Tensile strength is determined using conventional measurement techniquesemploying an MTI Phoenix universal test machine as described below. If aporous material is evaluated to determine tensile strength, the truecross sectional area of the sample must be taken into account bycalculating the matrix tensile strength. Matrix tensile strength iscalculated by multiplying the measured ultimate tensile strength by adensity ratio. The density ratio is the density of the copolymer (thedensity of the copolymer has a density of about 2.13 g/cc) divided bythe bulk density of the (porous) sample. For nonporous materials,tensile strength and matrix tensile strength are equivalent.

Purity Test

One of the possible impurities in the resulting resin is ammoniumperfluoro octanoate (APFO), which is the surfactant used during theemulsion copolymerization of the perfluoroelastomer. Chromatographytechniques such as gas chromatography (GC) can be utilized to detecttrace amounts of APFO that may still be present after the exhaustiveextraction process. Since APFO is a very polar compound, it adsorbs tothe GC column and gives a very broad and indistinguishable peak; thus,it cannot be directly quantified. In order to measure APFO, either afluorinated (polar) column is used or the APFO is converted to its lesspolar derivative. The latter method is more preferred. Equation 1(below) shows the methanolic/HCl derivitization reaction of APFO inwhich APFO is converted into its methylester form (MPFO).

This derivative is less polar and more suitable for undergoing analysisby GC-ECD. Electron Capture Detector (ECD) is used to substantiallyincrease the sensitivity of the analysis.

The purity of the copolymer (that is, resin or sheet) can be determinedby the following preferred procedure:

-   -   1) Ultrasonically mix 1-2 g of the material in 10-20 ml of an        appropriate extraction solvent (i.e., a solvent that dissolves        the analyte but not the matrix, such as MeOH) for 7-8 hrs at        60° C. (10-fold dilution).

2) Filter the material and use the effluent for the following steps.

3) Place 2 ml of the effluent (extract) in a reaction flask. Make 3repeats of each extract, and also use 2 ml of the extraction solvent asthe negative control.

4) Add 10 ml of 3N methanolic/HCl to each flask.

5) Cap and heat the flasks in a water bath at about 60° C. for 2 hrs.

6) Add 6 ml of n-hexane to each flask followed by 20 ml of NaCl solution(18%, 18 g NaCl in 100 ml of distilled water).

7) Shake the flasks vigorously and after the mixture settles remove theorganic (hexane) layer, and store it for GC-ECD analysis. Note that thepossible impurity concentration in each flask represents a 30-folddilution of its original concentration in the sample.

8) Analyze the samples by GC using the following parameters:

-   -   Column: HP-5 MS (cross-linked, 5% phenyl methyl siloxane, 25 m        length×0.25 mm ID×0.25 μm film thickness)    -   Ramp 1: 35° C. (5 min)-10° C./min-130° C. (1 min)    -   Ramp 2: 20° C./min-280° C. (5 min)    -   Injection Temp.: 240° C.    -   Detector Temp.: 300° C.    -   Injection Volume: 1 μl    -   Flow: 1 ml/min Direct Injection    -   Blank: Hexane    -   Run Time: 28 min

9) The concentration of APFO in the samples is then determined bycomparing the area under the peak of the methylated APFO, in theirrespective GC traces, to the calibration curve constructed from a seriesof standard or known MPFO solutions.

Resins produced from the preferred process as previously described andtested by the preferred purity test have exhibited purity levels as lowas below one part per million by weight. The preferred extractionprocess, when continued for extended times or simply repeated additionaltimes, or by additional purifying steps have resulted in improvedpurities, as low numerically as thirty parts per billion, which is thedetection threshold of the equipment used in the preferred purity test.Thus the actual purity levels may be numerically below this thirty partper billion level. More particularly, purity levels are attainable thatmay be below about 50 parts per million (ppm), 40 ppm, 30 ppm, 20 ppm,10 ppm, 5 ppm, 3 ppm, 2 ppm, 1 ppm, 500 parts per billion (ppb), 300ppb, 200 ppb, 100 ppb, 50 ppb and 30 ppb. If measurement accuracy andsample sizes permit, purity levels can be variously considered to bebelow 50 parts per million, 40 ppm, 30 ppm, 20 ppm, 10 ppm, 5 ppm, 3ppm, 2 ppm, 1 ppm, 500 parts per billion, 300 ppb, 200 ppb, 100 ppb, 50ppb and 30 ppb.

Tensile Test

The tensile properties of a noncross-linkable sample of the inventivecopolymer prepared in the form of a compression molded sheet and twosimilar commercially available elastomeric materials were determinedaccording to ASTM standard D638-94B using the type V dogbone sample.Eight to ten samples of each type were tested. Tests were conducted onan MTI Phoenix universal test machine equipped with an Instron® model2603-080 long travel extensometer for strain measurement. The sampleswere held with Instron® model 2712-003 pneumatic grips with hard rubberfaces (Instron® #2702-015). After attaching the extensometer to thenarrow portion of the dogbone, the sample was pulled to failure at aconstant cross head speed of 100 mm/min, corresponding to an initialstrain rate of approximately 650%/minute based on extensometermeasurements. Values for maximum stress, strain at break and 100% secantmodulus were calculated according to the ASTM standard using MTI Phoenixautomated analysis software. For calculations of stress and modulus, thethickness of each sample was measured using a Starrett snap gauge (0.002mm resolution). All materials tested were between 0.2 and 0.3 mm thick.Test data are presented in Table 1 in comparison to silicone andfluoroelastomer sheets. Sheets of Nusil platinum cured medical gradesilicone rubber of 0.25 mm thickness were fabricated by SpecialtySilicone Fabricators, Inc. of Paso Robles, Calif. Daiel T530 is afluoroelastomer sheet made by Daikin Industries Ltd., Osaka, Japan.TABLE 1 Tensile properties. Values for inventive copolymer sample,Silicone-Nusil Property +/−std dev MED4065 Daiel T530 Tensile 65.0 +/−13.3 8.78 +/− 0.7  52.0 +/− 8.7  Strength - MPa Elongation at 251 +/−34   928 +/− 22.7 614 +/− 160 Break - % 100% Secant  7.0 +/− 0.30 3.05+/− 0.05 3.0 +/− 0.5 Modulus - MPa

Tensile strengths which may be achieved with the copolymer of thepresent invention include matrix tensile strengths of about: 5, 7, 10,15, 20, 35, 50, 70, 85, 90, 95 MPa and above.

It is apparent that the copolymer of this invention has unexpectedsuperior tensile strength in comparison to the commercial products.

Copolymer Identification (NMR)

The TFE and PMVE copolymer of the present invention can be identified bynumerous conventional analytical means, for example by Fourier TransformInfrared (FTIR) Spectroscopy, or by Nuclear Magnetic Resonance (NMR)Spectroscopy. These example test methods, or other methods, can be usedto identify the primary copolymer composition (TFE and PMVE) of thepresent invention. Samples of the present invention have been analyzedusing Magic Angle Spinning (MAS) NMR Spectroscopy by Gleason Labs,Cambridge, Mass. Following are details relating to the specific testmethod used by Gleason Labs and typical results of such analysis.

1. Equipment

The equipment used is summarized in Table 2. TABLE 2 Equipment requiredfor NMR spectroscopy. Equipment Characteristics Chemagnetics 270 MHzDouble Serial no. PRB 270-395/6274 Resonance Solids Probe 3.2 mm MASspinning module Made of Vespel X channel Observe nuclei ranging from¹⁵N, ¹³C, ²⁹Si to ³¹P H channel Observe nuclei ¹⁹F and ¹H Operatingtemperature −150 to +250° C. Fiberoptic spin sensor Chemagnetics 3.2 mmRotor Parts Sleeve Made of zirconia (part no. SPA005-003) Drive tip Madeof Torlon (part no. SPA005-038) Bottom spacer Made of Vespel (part no.SPA006-051) End cap Made of Vespel (part no. SPA006-051) ChemagneticsSmart Speed Controller Serial no. ACC000-007/058 Chemagentics VT StackSerial no. ACC000-003/220 Tecmag Libra F-12 Data Acquisition Unit Serialno. M941222 Power supply Serial no. MP941223 Two-channel A/D boardDigitization rate of up to 10 MHz (100 ns) MacNMR PPC software Remoteacquisition and data analysis 6.3 T (270 MHz) Oxford Superconducting 89mm diameter wide bore Magnet JEOL room temperature shim unit2. Sample Packing.

The sample rotor consists of a sleeve, drive tip, bottom spacer, and endcap. The sample is positioned at the center of the rotor and representsa volume of 11 mm³. To pack the rotor, the drive tip and bottom spacerare first assembled onto the sleeve. Packing the sample then requiresthe use of a sample packing tool which has a marking to indicate themaximum level of filling that can be accommodated. The remainder of thevolume in the sleeve is then occupied by inserting the end cap. Powderedsamples are packed directly, while film samples have to be cut orcrushed to the correct dimensions before inserting into the sleeve.Several sequential steps of filling and packing were performed to reachthe maximum filling level. This ensured tighter packing and greatersample quantity, thus improving NMR detection sensitivity. Typically,sample weights of between 10-30 mg are achieved.

Powder free gloves are used to prevent extraneous contamination duringpacking. Suitable face masks are worn when appropriate to preventinhalation of airborne particulates. This is especially important forfilms which needs to be scraped off from a substrate before packing,since scraping may cause a substantial amount of particles to becomeairborne.

3. NMR Data Acquisition.

Acquisition is performed on a spectrometer using a Tecmag Libra dataacquisition unit and a 6.3 Tesla Oxford superconducting magnet.Correspondingly, Larmor frequency of ¹⁹F nucleus is 254.0 MHz. Toachieve high spectral resolution, a Chemagnetics double resonance solidsprobe capable of up to 25 kHz magic angle spinning is used to spin thesample rotor. Sample spinning is achieved by passing bearing and drivegas around the rotor which is placed inside the spinning module of theprobe. The drive gas is used to introduce rotational motion via contactwith the drive tip while the bearing gas is used to stabilize thespinning rotor. A Chemagnetics speed controller maintained speed towithin ±3 Hz of setpoint. Nitrogen gas is used as both the bearing anddrive gas to minimize probe contamination and to ensure spinningstability.

Typical acquisition parameters are given in Table 3. Total acquisitiontime for each one dimensional spectrum varies with sample relaxationcharacteristics, number of signal averages and the desiredsignal-to-noise ratio but normally is in the range of 10 to 30 min.TABLE 3 Typical NMR acquisition parameters. Parameter Typical ValuePulse length (90°) 1.25 ms Pulse power 100 W Spectral filter window ±100kHz Acquisition digitization rate 5 ms Number of signal averages  128Number of acquisition points 4096 Time between each signal average 10 sZero fill before Fourier transform x1 (8192 pts) Line broadening none4. Variable Temperature Setup.

Variable temperature (“VT”) studies between −150 to +250° C. can bedone. This is achieved by introducing heated nitrogen gas across therotor area during spinning. This nitrogen gas is a separate VT gas andis heated by a resistive heater within the VT stack above the probe. Athermocouple together with a temperature controller enabled gas inlettemperature to be controlled. Gas pressure required is not high, onlyabout 5 psig. The equipment and procedure listed above was replicatedfrom a Gleason Lab Standard Operating Procedure titled:

-   -   High Resolution Solid State 19F MAS NMR Spectroscopy.    -   Revised: 2.27.98 Gleason Lab, MIT.    -   Gleason Lab, Cambridge, Mass.

Typical results from the above analysis are shown in FIGS. 4A-4F. Shownin FIG. 4A is a NMR/MAS plot of “Intensity” vs. “ppm”, derived from theanalysis of the material of the present invention. The indication of, orsignatures of, three primary molecules are depicted by the threeintensity spikes 2, 4, and 6. Typical intensity spike 2, occurring atapproximately −55 ppm indicates the presence of the OCF₃ molecule. Thetypical spikes 4, centered at approximately −123 ppm, indicate thepresence of the CF₂ molecule. The typical spikes 6, centered atapproximately −138 ppm indicate the presence of the CF molecule. Thethree spikes 2, 4, 6 comprise a typical NMR/MAS signature of thenoncross-linkable copolymer (TFE and PMVE) of the present invention.

Similarly, FIGS. 4B through 4D show NMR/MAS plots of “Intensity” vs.“ppm”, for various random samples of the material of the presentinvention. Similar intensity spikes 2, 4, and 6, occur at approximatelythe same ppm as in FIG. 4A, indicating the presence of OCF₃, CF₂ and CFmolecules. The three combined spikes 2, 4, and 6, is a signature of thetwo monomers, TFE and PMVE of the present invention. As shown in FIGS.4A through 4D, the relative intensities of the spikes 2, 4, and 6 canvary. The signature, or indication of the presence, of the copolymer ofthe present invention is indicated by the location or ppm of theparticular intensity spikes.

The degree of resolution regarding a spike profile can be enhanced byraising the temperature of the sample under test. The increasedresolution of the signature spikes 4 and 6 is shown in FIG. 4E, whichcompares signatures of the present invention derived at 25 and 100° C.Similarly, the resolution of a signature spike can be enhanced byextended sequence resolution. The increased resolution of the signaturespikes 4 and 6 is shown in FIG. 4F, which compares signatures of thepresent invention derived at 25 and 100° C., while also incorporatingextended sequence resolution.

Copolymer Identification (FTIR)

The TFE and PMVE copolymer of the present invention can also beidentified by Fourier Transform Infrared (FTIR) Spectroscopy. Organiccompounds absorb infrared light at various wavelengths depending on thetypes of chemical bonds present in the molecule. An IR spectrum providesa fingerprint or signature of the organic compound which allowsconfirmation of the copolymer TFE and PMVE of the present invention. Thespectrum is a plot of the amount of light absorbed versus the wavelengthor frequency of infrared light. Samples of the present invention havebeen analyzed using internal Fourier Transform Infrared (FTIR)Spectroscopy. Following are details relating to the specific test methodused internally along with typical results of such analysis.

The following FTIR instruments are used in the analyses.

1) Perkin Elmer Model 1600 FTIR Spectrophotometer.

2) Nicolet 510P FTIR Spectroscopy, equipped with Nicolet PC/IR softwareversion 3.2 run on a Hewlett Packard Vectra 486/33T personal computer.

The FTIR instrument parameters are:

-   -   Scan Range: 4400 cm⁻¹-450 cm⁻¹    -   Resolution: 2.0 cm⁻¹    -   Apodization: Strong    -   Scan #: 16

Samples of the copolymer of the present invention are prepared foranalysis according to FIG. 5. Approximately 4 gms of the sample 14 areplaced between two high temperature sheets 12. The high temperaturesheets 12 and the sample 14 are then placed into a lower die 16 cavity.The upper die 10 is aligned to the lower die and placed into a heated,vacuum platen press (not shown). The die is heated to approximately 250°C. The die containing the sample is initially subjected to vacuum forapproximately two minutes, compressed by applying load 18 atapproximately 14 MPa for approximately 30 seconds and allowed tostabilize under vacuum for approximately 2 minutes.

FTIR Analysis

1. Latex gloves should be worn throughout the following analysis.

2. A background is scanned using FTIR parameters of 16 scans over therange of 4400-450 wavenumber (cm⁻¹) and the resolution of 2 wavenumbers.It takes 2 minutes for the analysis.

3. Samples of about 4 cm² are cut, the high temperature sheets areremoved using a pair of forceps, and the sample layer is placed into thesample holder in the instrument's sample chamber.

4. The sample chamber is purged out for 3-4 minutes prior to theanalysis.

5. The sample is then scanned using the above FTIR parameters.

6. The y-axis (absorbance) is scaled to 3.0 absorbance units, while thex-axis (wavenumber) is scaled from 450 to 3000. The baseline correctionand smoothing of the FTIR spectrum are then adjusted.

7. Spectrum of a copolymer of the present invention shows two featurepeaks, 2360 cm⁻¹ and 890 cm⁻¹, which are assigned to TFE and PMVEregions, respectively.

8. The absorbance height at peak of 2360 is determined over the range of2800-2200 cm⁻¹, while that of 890 is calculated over the range of about921-875 cm⁻¹.

9. Comments are inserted onto the spectrum, and the two peaks ofinterest are labeled.

10. The resulting spectrum is then plotted.

11. The following equation is utilized to calculate PMVE wt % of thecopolymers.${{PMVE}\quad{wt}\quad\%\quad{of}\quad X} = {\frac{R_{x}}{R_{ref}}\quad( {{PMVE}\quad{wt}\quad\%\quad{of}\quad{Reference}\quad{IR}} )}$where X=copolymer; R=(A₈₉₀/A₂₃₆₀); A=absorbance. The reference materialcan be any copolymer of TFE and PMVE whose composition has beendetermined by another method, for example NMR.

Typical FTIR results, derived from the analysis of samples of thepresent invention are shown in FIG. 6. Shown is a graph of “absorbance”vs. “wave number,” typical of a FTIR analysis. As shown in FIG. 6, thecopolymer of the present invention has three significant absorbancespikes 28, 30 and 32. The first absorbance spike 28 is typicallycentered upon the wave number datum 20, occurring at approximately 2360cm⁻¹. The second absorbance spike 30 is typically centered between thewave number datums 22 and 24 occurring at approximately 1350 and 1100cm⁻¹ respectively. The third absorbance spike 32 is typically centeredupon the wave number datum 26, occurring at approximately 890 cm⁻¹.

Optical Transmittance

The optical properties of the copolymer of the present invention havebeen quantified by Optical Data Associates, Tucson, Ariz. Totalreflectance was measured using a Cary 5E Spectrophotometer and thenverified using a Metricon Prism Coupler. Following are the derivedrefractive indices for three random samples of the copolymer of thepresent invention: Spectrophotometer, 630 nm Metricon, 633 nm Sample 11.341 1.326 Sample 2 1.322 1.326 Sample 3 1.313 1.330

Shown in FIG. 7 is a derived graph of “Diffuse Transmittance” vs.“Wavelength” for the three optical test samples of the copolymer of thepresent invention referred to above. Shown in this Figure is thebaseline 34, along with the representative transmittance values 36, 37and 38 for the samples tested. As shown in FIG. 7, the copolymer of thepresent invention displays a light transmission of 90% or greater, or94% or greater over a wide spectrum of wavelengths.

Frictional Wear

The copolymer of present invention was tested to determine thefrictional wear or abrasion characteristics, using commerciallyavailable, thermoset silicone as a control comparison. Abrasion or wearresistance was evaluated using an AMTI, Advanced Mechanical Technology,Inc., Waltham, Mass., Abrasion tester, 1997-1998 model, six specimenOrtho-Pod tester. Details of this abrasion tester are shown in FIG. 8A.Shown in FIG. 8A is a cross section of a typical sample fixture 40. Thetest sample 44 is placed into the sample fixture 40. A ring clamp 43 isplaced onto the sample 44 and secured by applying a clamping load 43.The sample fixture 40 is filled with deionized water 46. A 9 mmdiameter, ceramic aluminum oxide abrasion pin 48, having acircumferential edge radius 50, is then approximately aligned to thecenter axis 54 of the sample fixture 40. During testing, the abrasionpin 50 is forced against the test sample 44 by applying load 52. Shownin FIG. 8B is an isometric view of the abrasion wear tester. Up to six,water filled, sample fixtures 40, containing clamped samples to betested, are fixtured onto an oscillating base plate 56. During testing,the base plate 56 oscillates 58, about a central axis 64. Up to sixabrasion pins 48 are approximately aligned to the central axis 54, ofthe sample fixtures 40. The individual abrasion pins 48 oscillate 60,about their central axis 54. The applied load 62 is also varied duringtesting. During a typical wear abrasion test, the base plate 56oscillates at approximately 2 Hz. The angular displacement of the baseplate 56 is adjusted to contain the abrasion pins 48 within the innerdiameter or opening of the sample clamping rings. Typical angulardisplacements of the base plate 56 result in a 15 to 18 mm lineardisplacement of the abrasion pin relative to the test sample. Theabrasion pins are rotationally oscillated, approximately ±20 degrees, atapproximately 4 Hz. The abrasion pin load 62, is varied between 7±2 Kg,at a frequency of approximately 2 Hz.

Abrasion or wear characteristics are determined by subjecting thecomparative material samples to a number of abrasion cycles and thenmeasuring the sample weight loss. The percent change in sample weight isthen calculated. An abrasion cycle is defined as one completeoscillation, or one full angular excursion of the base plate. Followingis typical abrasion wear data comparing materials of the presentinvention to commercially available thermoset silicone. The siliconeused for the comparison was MED-4716/PT MDX4-4516, procured from NuSilSilicone Technology, Carpinteria, Calif.

Number of Abrasion Cycles: U.S. Pat. No. 4,804,688 % Weight ChangePresent Invention 1 0.06 Present Invention 2 0.12 Present Invention 30.22

Number of Abrasion Cycles: U.S. Pat. No. 2,007,666 % Weight ChangeSilicone Sample 1 6.74 Silicone Sample 2 7.40 Silicone Sample 3 0.45

The comparative silicone samples abraded more rapidly than the samplesof the present invention, so the testing of the silicone was terminatedat a lower number of abrasion cycles.

As seen by this comparative data, the thermoplastic copolymer of thepresent invention exhibited approximately 85 times less abrasion wear,compared to thermoset silicone, based on weight change averages and anapproximated equal number of abrasion cycles.

Tensile Set

The material of the present invention was analyzed to determine tensileset properties. Values of tensile set were determined for varioussamples of the present invention using the method outlined in ASTMStandard D412 in which the material is strained to a fixed value (100%)and held for 10 minutes, then released and allowed to recover for 10minutes. The amount of permanent deformation in the sample is determinedby measuring the distance between benchmarks on the narrow portion ofthe dogbone sample. The permanent set is then expressed as a percentageof the original length. It was discovered that materials of the presentinvention, with relatively high amounts of the PMVE monomer (65 to 70 wt%), possessed remarkably low average tensile set values of less than0.9%.

Suture Hole Leakage

The elastomeric properties of the thermoplastic copolymer of the presentinvention are useful for a wide variety of applications. For example,elasticity can be imparted to devices imbibed or coated with thecopolymer of the present invention. The elastomeric properties can alsobe used to at least partially seal holes, for example, suture orcannulation needle punctures. By laminating or hot melt reflowing thecopolymer of the present invention onto a vascular graft or surgicalpatch, the perforation leakage is greatly reduced due to the elastomericbehavior of the thermoplastic material which recovers and seals at leastpartially the puncture.

In order to evaluate and quantify the reduction in suture hole leakage,as a result of the incorporation of the material of the presentinvention into a vascular graft or patch, an in vitro test has beendeveloped. This in vitro leak rate test is depicted in FIG. 9A. As shownin FIG. 9A, an air over fluid reservoir 72, containing filtered,deionized water 74 is pressurized by applying air 70 at a constant 5.9KPa. The water is maintained at a constant 40° C. The test sample isprepared by placing through the sample a specific number of sutureshaving a specific pattern. The prepared sample 78, is then positionedinto a leak fixture 76. The sample 78 is clamped by a clamping ring 80which is secured by clamping load 82. The constant water pressure,delivered to the upper surface of the test sample, forces water throughthe openings 84, created by the pre-suturing. Water droplets 86 arecaptured by a collection funnel 88 and directed into a graduatedcylinder 90. The volume or weight of the collected water 92, over aspecific amount of time, is used to calculate a leak rate, in ml/minute.This in vitro test defines the average water leak rate for a sampleprepared in accordance with the following sample preparation procedure.The leak rate of various sample materials may be determined and comparedusing this test. The test sample is prepared for the invitro leak testaccording to FIG. 9B.

Shown in FIG. 9B is a top view of the test sample 78. A continuoussuture pattern is placed between the two diameters 94 and 96. The sample78 is clamped between diameters 98 and 100. The diameter 94 isapproximately 2.10 cm, the diameter 96 is approximately 2.54 cm, thediameter 98 is approximately 2.80 cm and the diameter 100 isapproximately 4.20 cm. As shown in FIG. 9C, a series of marks 102 areprinted onto the sample. Forty marks 102 are printed, evenly spacedalong each diameter 94 and 96. The marks 102 are paired or oriented inan approximate radial fashion, such that a line projected through a pairof marks crosses the approximate center of the sample. The sample isthen sutured with a single 3-0 Braided Silk suture on a RB-1 needle(Ethicon Part Number K872H). Using the marks 102 as a guide, acontinuous suture 104 is threaded through the sample and through a pairof marks without tension. The suture placement is continued, withouttension through the sample and through all remaining marks. Uponcompletion of the pattern, the suture is knotted using four flat squarethrows.

EXAMPLES Copolymer Preparation

Without intending to limit the scope of the present invention, thefollowing examples illustrate how the copolymer of the present inventionmay be made and used.

Example 1

A 2 liter pressure vessel is subjected to vacuum and purge with TFE gasthree times to reduce oxygen content to below 30 ppm. 1000 g ofdeionized water, 50 g of PMVE monomer, and 90 g of ammoniumperfluorooctanoate surfactant (Fluororad FC-143, 3M Corp., Minneapolis,Minn.) are added. The mixture is a transparent microemulsion at 50° C.and is maintained at a stirring speed of about 1200 rpm. Then thetemperature of the mixture is raised and maintained to be about 75° C.,and tetrafluoroethylene gas is charged to the reactor and the pressureinside the reactor is about 1500 KPa. 0.4 g of ammonium persulfateinitiator in 40 g of water is pumped into the reactor to start thereaction. Reaction proceeds for about 42 minutes and is stopped. At theend of the reaction, the pressure inside the reactor is about 200 KPa,which means that a sufficient amount of tetrafluoroethylene was usedduring the polymerization reaction.

Example 2

In a 10 litre stirred pressure reactor, 5.5 kg of deionized water and150 g of Fluorad® FC143 (3M produced ammonium perfluoro octanoate) werecharged, under a constant agitation speed of 900 rpm at ambientcondition. Subsequently, the reactor was purged with tetrafluoroethylenegas, following with a vacuum. This was repeated three times to insurethe oxygen level in water is below 30 ppm. Then, 510 grams of a liquidperfluoro methyl vinyl ether monomer (CF₂═CFOCF₃) was charged to thereactor under the same agitation speed. It is important to charge themonomer as a liquid in order to form a microemulsion in water. Then, 550grams of gaseous tetrafluoroethylene monomer (CF₂═CF₂) was charged tothe reactor. At this moment, the reactor was fully sealed and the insidetemperature was raised to 70° C. under the same agitation speed. Thepressure inside the reactor was recorded at 1.9 MPa. Then 5 grams ofammonium persulfate, a free radical initiator dissolved in 100 grams ofdeionized water, was pumped into the reactor to start the microemulsionpolymerization. The reaction temperature was maintained at about 70° C.during the polymerization and the pressure inside continued to drop aspolymerization proceeded. From the beginning of the initiator charge toa point where pressure inside the reactor dropped to 0.4 MPa was a totalof 53 minutes. Reaction was stopped at this point by evacuating thereactor and cooling the reactor temperature to ambient condition. Atotal of about 7 kg of a transparent aqueous dispersion was formed,wherein the resulting copolymer solid content was about 12.4% and thenumber average copolymer colloidal particle size was about 33 nanometerdetermined by a laser light scattering technique. The composition of theobtained copolymer of (tetrafluoroethylene and perfluoro methyl vinylether) was determined by FTIR to be about 54% by weight ofperfluoromethyl vinyl ether. The copolymer was found to have tensileproperties of 61 MPa in tensile strength and 248% in tensile elongationand a density of 2.144 g/ml.

EXAMPLES Other

The polymer so produced can be applied directly from the colloidaldispersion by immersing a substrate material into the dispersion, or bypainting the substrate with the dispersion, or by spraying thedispersion onto the substrate. Suitable substrates include variousporous and nonporous substrates, including fabrics, woven or non-wovenmaterials, screens, paper, or porous or microporous membranes of anyform including sheets or tubes. Once the coating is applied to thesubstrate, any water, surfactant or initiators remaining can be drawnoff by any convenient means, such as heating, steam stripping, vacuumevaporation, or the like.

A particularly preferred porous substrate is a microporouspolytetrafluoroethylene made by stretching polytetrafluoroethylene asdescribed in U.S. Pat. Nos. 3,953,566 and 4,187,390 to Gore,incorporated by reference. In this procedure the structure comprisespolymeric nodes interconnected by fibrils, the nodes and fibrilscomprising a microstructure defining pores or void spaces therein.

As shown in FIG. 10A cross section, a laminated structure may be formedcomprising a first layer 106, a second layer of the inventive copolymer108 and a third layer 109. The layers 106, 108, 109 have respectivethickness 110, 112, 114 and the laminate has a total thickness 116. Thefirst and third layers 106, 109 can be comprised of any materialssuitable for the intended purpose of the laminated device. The firstlayer 106 and the third layer 109 can consist of the same, similar ordifferent materials. For example, the material of the present inventionor layer 108 may be used to simply bond the first layer 106 to the thirdlayer 109. An example of this configuration is a hernia repair patch,wherein the first layer 106 is comprised of a material which encouragestissue attachment, while the third layer 109 is comprised of a materialwhich discourages tissue attachment.

Another embodiment of the laminated device shown in FIG. 10A is asurgical sheet which can be used as a surgical patch, or a dura matersubstitute having reduced suture hole bleeding or leakage. Theelastomeric properties of the inventive copolymer cause the layer 108 torecover from being punctured. The suture hole is therefore at leastpartially sealed and bleeding or leakage is reduced. A low bleedsurgical sheet can be formed from a 0.4 mm thick GORE-TEX®Cardiovascular Patch, available from W.L. Gore & Associates, Inc.,Flagstaff, Ariz. The 0.4 mm thick patch can be slit into twoapproximately equal thickness of about 0.2 mm. The copolymer of thepresent invention is formed into a sheet, approximately 0.2 mm thick bythermo compression. Typical compressive loads are approximately 14 MPawith typical temperatures of about 250° C. The sheet of the inventivecopolymer is then placed between the two layers of the patch materialand the three layers are then thermo compressed to form the laminate.Typical lamination temperatures are around 200° C. with appliedpressures less than 7 KPa, for approximately 7 minutes. The first layer106 has a typical thickness 110 of about 0.2 mm, the second layer 108has a typical thickness 112 of about 0.2 mm and the third layer 109 hasa typical thickness 114 of about 0.2 mm. The resultant surgical sheethas a typical total thickness 116 of about 0.6 mm. Devices of thislaminated configuration, incorporating the material of the presentinvention, display average leak rates of less than about 6 ml/minute,when subjected to the water leak test previously described.

Dura mater substitutes can be formed in a similar fashion. The first andthird layers 106 and 109 can consist of expanded polytetrafluoroethylene(ePTFE), as previously described. Thickness 110, 112, 114 of the threelayers 106, 108, 109, can be tailored to optimize handling and leakrate. The specific microstructure of the first and third layers can bevaried to enhance tissue response and optical clarity. Dura matersubstitutes having first and third layers about 0.1 mm thick withaverage microstructures of about 5 microns, incorporating a second layerabout 0.2 mm thick of the inventive material, display average leak ratesof less than 9.0 ml/minute, when subjected to the water leak testpreviously described.

As shown in FIG. 10B, the laminated structure can have layers ofdifferent thickness 110, 114.

As shown in FIG. 10C the copolymer of the present invention 108 can belaminated or bonded to another material 106 by conventional thermocompression means. Laminate assemblies are not limited to two or threelayers and can include 4, 5, 6, 7, 8, 9, 10 or more layers, comprisingthe same, similar or different alternating materials.

As shown in FIG. 10D, a layer 106 of a laminated device can have indents118, for example to encourage tissue attachment or to capture or containother materials, such as antimicrobials or other therapeutic agents. Theregion 119 below the indent 118 can be densified to enhance the opticalclarity or light transmission, particularly in the use of dura matersubstitutes.

As shown in FIG. 10E, the indent 118 can penetrate the entire thicknessof a layer 106.

Shown in FIG. 10F is a partial cross section of a fluid container 115.The container walls incorporate the material of the present inventionand contain a fluid 117.

Being a thermoplastic with a relatively low melting temperature, thecopolymer of the present invention can be readily injection orcompression molded into a variety of shapes. For example, as shown incross section FIG. 11A, the inventive material can be molded into animplantable lens 120 or cornea. FIG. 11B depicts an optional crosssection of an implantable lens. Other molded shapes can include, but arenot limited to O-rings, gaskets, implantable augmentation or spacefilling devices, joint spacers, pump diaphragms, filters or other, longor short term, implantable device.

Two methods have been developed for using the copolymer of the presentinvention to bond porous materials. Both rely on forming a mechanicalinterlock within the pores of the material to be bonded. The firstbonding method is by thermal bonding. In this method the material of thepresent invention is compression molded into a thin sheet (0.05 to 0.3mm thick) and placed in contact with the porous material. The constructis then placed under light pressure (approximately 0.7 KPa) and heatedto about 200° C. for 5 minutes. For the porous PTFE materials used todate the resulting bond is stronger than the base material. Forinstance, in peel tests of a laminate of the material of the presentinvention and GORE-TEX® cardiovascular patch (available from W. L. Goreand Associates, Flagstaff, Ariz.), cohesive failure of the PTFE occursat approximately 1.45 N/cm. Therefore the bond strength for this exampleis greater than 1.45 N/cm.

The second method is by solution bonding. The copolymer of the presentinvention can be dissolved in Fluorinert FC-75 solvent (3M Corp,Minneapolis, Minn.) in concentrations up to approximately 10%. Thissolution can be spread onto porous materials which can then be bonded byapplying light pressure while the solvent evaporates.

The use of the inventive copolymer as a bonding agent is illustrated inFIGS. 11C and 11D, which show similar or indifferent materials 124,bonded together. The inventive material or bonding agent 126, can beused to bond materials 124 in various bond configurations, for examplelap joints or end-to-end joints shown respectively in FIGS. 11C and 11D.

The thermoplastic copolymer of the present invention can be meltextruded into a variety of continuous shapes such as rods, tubes orshapes having non-circular cross sections such as triangles, squares, orany polygon. Such shapes are suited for use as long or short termimplantable catheters, urinary catheters, non-implantable catheters,medical, peristaltic pumps, or food grade tubing, sutures or other, longor short term, implantable devices. Shown in FIG. 12A is a single lumen129 catheter 128 consisting of the inventive material. FIG. 12B depictsa multi lumen 129 catheter 130, extruded from the material of thepresent invention.

FIG. 12C shows a multilayer tube 131, having a lumen 129, an inner wallor layer 134, and an outer surface or layer 132, comprised of theinventive copolymer. Shown in FIG. 12D is a similar configuration of atube 133, having a lumen 129, with an inner wall or layer 132 comprisedof the inventive copolymer and an outer wall or layer 134.

Shown in FIG. 12E is a three layer tube 135 having an inner lumen 129,an inner wall or layer 134, a internal layer 132 of the inventivecopolymer and an outer wall or layer 134. Tubing configurations are notlimited to these depicted configurations and may include tubes having 4,5, 6, 7, 8, 9, or 10 layers or walls. Tubes of these or similarconfigurations can be co-extruded by conventional means or separatelyextruded then subsequently joined or bonded together, film wrapped thenbonded together, or by any other suitable means. Billets can also beformed having concentric layers of the dissimilar materials andconcurrently extruded. The thermoplastic nature of the inventivecopolymer facilitates the joining of terminations or fittings onto thetubing. The thermoplastic copolymer of the present invention may also beused as a processing aid (e.g., extrusion aid) to facilitate theprocessing of PTFE. The noncross-linkable copolymer of TFE and PMVE canbe blended to PTFE in either dispersion form or dry powder form. Thecopolymer and PTFE blend can be co-coagulated (from dispersion) to forma coagulum; or dry blended. The resulting blend can thus be fed into anextruder at temperatures between about 500 and 350° C. The resultingextrudate can be further processed by expansion methods to impart nodeand fibril structures.

The copolymer of the present invention can be hot melt extruded intorods, fibers, threads or cylinders, or for example, a suture as shown inFIG. 12F. In the application of a suture 136, the thermoplastic natureof the present invention greatly facilitates the attachment of a needle.

Shown in cross section FIG. 13A is a mesh 137 comprised of the copolymerof the present invention. Conventional methods can be employed to formfibers, weave fibers into various mesh patterns and, due to thethermoplastic properties of the material, be subsequentlythermo-compressed to join or bond the fibers. Shown in FIG. 13B is asimilar mesh 136, bonded to another material or substrate 138.

The copolymer of the present invention can be used as a biocompatible,ultra pure surface coating, applied by hot melt dipping or spraying,vacuum thermo-lamination or by applying forms of the inventive materialonto a substrate and then thermo-reflowing. Shown in FIG. 13C is asubstrate 140, surface coated with the inventive material 142. Thecoating can have voids or pores 144, open surface areas 148 and varyingcoating thickness 146. If applied to a porous substrate surface 141, thematerial of the present invention 142 can serve as a pore filler orsealer. The material of the present invention can also be imbibed intoporous substrates, thereby altering the material characteristics of theinitial substrate.

The copolymer of the present invention can be rendered porous by avariety of conventional means which include, but are not limited to,gas, liquid or powder foaming, pre-imbibing, for example salts, into thematerial and subsequently dissolving the salts, by forcing a heated gasthrough the softened material or by mechanical or laser perforating.Shown in FIG. 13D, is a cross section of the inventive material 142,having pores or voids 150 of various shapes, angles and sizes, alongwith pores or voids having varying cross sections 152 along theirlength. As shown in FIG. 13E, the material of the present invention 142,having pores or voids 150 can be subsequently bonded or attached to asubstrate 154.

Shown in FIG. 14A is a foamed open cell 160 cross section of theinventive material. Shown in FIG. 14B is a foamed closed cell 162 crosssection of the inventive material.

FIG. 14C depicts a cross section of the material 164 of the presentinvention after impregnation with the same, similar or dissimilarmaterial 166.

Shown in FIG. 15 is a node 170 and fibril 172 structure, for examplethat of expanded PTFE, imbibed with the material 174 of the presentinvention.

FIG. 16 depicts a cross section of a bone joint spacer 176, havinglocating stems 178 consisting of the material of the present invention.

Shown in FIG. 17A is a cross section of a stent support structure 180,coated with the material 182 of the present invention. FIG. 17B shows across section of a stent support structure 180, wherein the stent isexternally covered with the material 182 of the present invention.Similarly, as shown in FIG. 17C the stent support structure 180 can beinternally coated with the material 182 of the present invention.

Shown in FIG. 18 is a typical stress strain to break profile formaterials of the present invention. The stress strain curve 200 ofmaterials typical of the present invention, has an ultimate tensilestrength 204 and a strain to break 206.

Shown in FIG. 19 is a typical stress, strain to 100%, profile showingthe recovery or resilient characteristics for materials of the presentinvention. The stress strain curve under load 208 continues to 100%strain 212. The stress strain curve under relaxation 210, continues tozero stress, where the residual strain 214 can be quantified.

While particular embodiments of the present invention have beenillustrated and described herein, the present invention is not limitedto such illustration and description. It should be understood thatadditional embodiments may be envisaged and incorporated withoutdeparting from the scope of the present invention as defined by thefollowing claims.

1. An article comprising a copolymer of tetrafluoroethylene andperfluoro(methyl vinyl ether); wherein the copolymer is a thermoplastic,wherein the copolymer contains between about 40 and 80 weight percentperfluoromethyl vinyl ether and complementally 60 and 20 weight percenttetrafluoroethylene, wherein said copolymer has a matrix tensilestrength greater than about 35 MPa.
 2. The article of claim 1 whereinthe copolymer has a matrix tensile strength greater than about 50 MPa.3. The article of claim 2 wherein the copolymer has a matrix tensilestrength greater than about 70 MPa.
 4. The article of claim 3 whereinthe copolymer has a matrix tensile strength greater than about 85 MPa.5. The article of claim 4 wherein the copolymer has a matrix tensilestrength greater than about 95 MPa.
 6. The article of claim 1 whereinsaid article is an implantable article.
 7. The article of claim 2wherein said article is an implantable article.
 8. The article of claim3 wherein said article is an implantable article.
 9. The article ofclaim 4 wherein said article is an implantable article.
 10. The articleof claim 5 wherein said article is an implantable article.